“These talks relate to an extremely sensitive topic, so it is premature to talk about it now. Many people observed Erdogan’s stay in Zhukovsky, what he looked at, what we showed. Both the Turkish side and our side have interest in this,” Ushakov told reporters in Moscow, adding that another Russian-Turkish contract on the delivery of the S-400 air defense systems is close to completion.

Russian President Vladimir Putin is heading to Ankara on Monday to attend a trilateral summit on Syria.

Bilateral meetings are also expected on the occasion, Ushakov said, where military cooperation is an agenda for the meeting between Erdogan and Putin.

“At the bilateral meeting, the leaders, in addition to Syria, will discuss some important questions of the implementation of the agreements that were reached in Moscow and cooperation in the military-technical domain,” he said.

With Iranian President Hasan Rouhani, Putin will discuss the establishment of an alternative payment to SWIFT. The issue is gaining momentum as Russian oil companies show interest in Iranian oilfield development.

He added that the trilateral summit could pave way for a quadrilateral summit with participation of Turkey, Russia, Germany and France.

Turkey, Russia, and Iran are the guarantor countries that brokered a cease-fire in Syria in December 2016, leading to the Astana, Kazakhstan talks, which are running parallel to the Geneva peace talks.

Syria has been locked in a vicious civil war since early 2011, when the Assad regime cracked down on protesters with unexpected ferocity.

Since then, hundreds of thousands of people are believed to have been killed and millions more displaced by the conflict.